High energy forming process



United States Patent 3,156,039 HIGH ENERGY FORMING PROESS Charles H. Martens, Huntsville, Ala, assignor to the United States of America as represented by the Secretary 0f the Army N0 Drawing. Filed Sept. 24, 1963, Ser. No. 311,259

4 Claims. (61. 29-421) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein'may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The invention relates to an improvement in the high energy forming processes used in the shaping of metals. More particularly, the invention relates to improving high energy metal forming processes by using low-temperature fluids as a medium for transmitting pressure in lieu of Water and other commonly used fluids such as air.

The present application is a continuation-in-part of applicants copending application Serial Number 283,638, filed May 27, 1963.

The terminology high energy forming process as used herein refers to those methods of forming, shaping, sizing, and flanging of metals whereby a source of energy almost instantaneously develops tremendous pressure in a fluid medium. This includes explosive forming and capacitor discharge elec-trospark forming but is not limited thereto. Generally a tank is filled with water and a female die, with a metal work piece held rigidly in place over the die cavity, is immersed in the tank. The die in most instance is firmly held stationary in the tank. An explosive charge is detonated in the Water creating tremendous pressures which are transmitted to the work piece forcing it to conform to the shape of the die cavity. The process is discussed in detail by John Pearson in the Journal of Metals, vol. 12, No. 9, September 1960, pages 673-681.

According to the present invention, the metal work piece to be formed by the high energy forming methods of the prior art is maintained at a temperature of l00 F. or less at the time the pressure is applied. It has been found that the lower temperatures impart greater strength to the metals thus formed with the strength of the austenitic stainless steels being particularly improved. A preferred embodiment of the invention is the high energy forming of A181 301 stainless steel sheet while the steel is maintained at the temperature of liquid nitrogen (320.4 R).

In accordance with the foregoing, it is an object of the present invention to provide an improvement in the high energy forming of metals by maintaining the metals at low temperatures.

It is a further object of the present invention to provide an improvement in the high energy forming of austenitic stainless steels by maintaining the steels at the temperature of liquid nitrogen during the forming process.

A still further object is to provide an improvement in the high energy forming an A181 301 stainless steel sheet by maintaining the sheet at the temperature of liquid nitrogen during the forming process. 7

The manner in which these and other objects may be accomplished will become apparent from the following description.

The advantages of high energy forming are well known in the art. Close tolerances and negligible spring-back characterizes items prepared in this manner. Moreover, extremely large parts as well as small parts of complicated design can be fabricated in this manner. In most instances,

it is not necessary to preform the work piece to roughly conform to the shape of the die since ordinary flat sheet metal works satisfactorily. Furthermore, the process is economical by virtue of its simplicity, especially when making only a limited number of parts.

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Explosives such as dynamite, smokeless powder, black powder, trinitrotoluene, pentaerythritol tetranitrate, and the like in bulk, sheet, cord, or liquid form are normally used to develop the tremendous pressures used in high energy forming. Pressures of up to 4,000,000 p.s.i. can develop in a period of time measured in microseconds. Although explosives such as black powder may develop pressures of only 40,000 p.s.i. or less and require several milliseconds. However, in lieu of explosives, capacitor discharge electrospark forming can be used. In this case, a charge of 35,000 volts to 100,000 volts is stored in a capacitor. The capacitor is then caused to discharge across two electrodes immersed in the water. As a result of the discharge, tremendous pressure is transmitted to the work piece through the water.

The dies used in high energy forming are generally made of metals although hard rubber and plastics, particularly Teflon, are also used. For use with the present invention, any conventional dies are acceptable. However, due to the brittleness of some die materials at low temperatures care should be exercised in selecting dies for very cold work. Dies of steel will normally be utilized.

The process of high energy forming metals is greatly improved if the work piece is maintained at a temperature of l00 F. or less when it is formed. This is readily accomplished by immersing the work piece and die in a fluid medium which is maintained at temperatures of F. or less rather than by immersing the work piece and die in water. The cold fluid transmits the pressures evenly just as does water. Suflicient time should be allowed for the temperature of the metal work piece to be lowered to at least -100 F. It is desirable to allow the temperature of the work piece to come to equilibrium with the temperature of the cold fluid.

The advantage of low temperature high energy forming is particularly noticeable when forming sheets of austenitic stainless steels. When a work piece of AISI 301 stainless steel sheet is explosively formed at the temperature of liquid nitrogen (320.4 F.), the item produced is measurably stronger than an item produced in a water medium. The additional strength imparted to the steel by high energy forming at low temperature makes the process especially valuable in the fabrication of parts for use in missiles and rockets where increases in strength and/ or reduction in weight are critical considerations.

All that is required to make use of the present invention is to substitute a fluid having a temperature of l00 F. or less for the water, air, and the like presently used as the pressure transmitting fluid in high energy forming process now in use. Since liquid nitrogen is inert, it is the preferred medium-for use in the improved process of the present invention. For extremely low temperatures, liquid helium can be employed. However, if the pressures are transmitted to the fluid by a piston so that there is no direct contact of the fiuid with an explosion or electrical discharge, liquid hydrogen, liquid oxygen or a Dry ice-acetone mixture can also be used. The work piece should be allowed to stand in contact with the low temperature fluid for a period of time suflicient to insure cooling to at least -l00 F. and preferably to the temperature of the fluid before forming takes place. After the forming operation has taken place, the work piece is removed from the die and is ready for any further processing such as heat treatment.

Since liquid nitrogen and other liquid gases are extremely volatile, provisions must be made for venting gases. However, since most high energy forming processes are conducted with the pressure transmitting fluid open to the atmosphere, venting gases is no problem.

No undue limitation should be attributed to the invention by the above description except as reflected in the appended claims.

I claim:

1. In the high energy forming of austenitic stainless steels whereby high pressures are instantaneously created in a fluid and thereby transmitted to the surface of a work piece forcing it to conform to the configuration of a die, said Work piece and said die being immersed in said fluid; the improvement which comprises substituting for said fluid a liquid having a temperature of 100 F. or less and allowing the temperature of said work piece to come to equilibrium with the temperature of said liquid before forming said work piece.

2. In the high energy forming of austenitic stainless steels whereby high pressures are instantaneously created in a fluid and thereby transmitted to the surface a work piece forcing it to conform to the configuration of a die, said work piece and said die being immersed in said fluid; the improvement which comprises substituting liquid nitrogen for said fluid and allowing the temperature of said work piece to come to equilibrium with the temperature of said liquid nitrogen before forming said work piece.

3. In the high energy forming of A181 301 stainless steel sheet whereby high pressures are instantaneously created in a fluid and thereby transmitted to the surface of a work piece forcing it to conform to the configuration of a die, said work piece and said die being immersed in said fiuid; the improvement which comprises substituting liquid nitrogen for said fluid and allowing the temperature of said work piece to come to equilibrium with the temperature of said liquid nitrogen before forming said work piece.

4. In the explosive forming of a work piece of A151 301 stainless steel sheet whereby high pressures are instantaneously created in a fluid by detonation of an explosive charge therein, said pressures thereby being transmitted to the surface of a work piece forcing it to conform to the configuration of a die, said work piece and said die being immersed in said fluid; the improvement which comprises substituting liquid nitrogen for said fluid and allowing the temperature of said work piece to come to equilibrium with the temperature of said liquid nitrogen before detonating said explosive charge.

Ziegler et al. Oct. 24, 1950 Howlett et a1. Jan. 31, 1961 

1. IN THE HIGH ENERGY FORMING OF AUSTENITIC STAINLESS STEELS WHEREBY HIGH PRESSURES ARE INSTANTANEOUSLY CREATED IN A FLUID AND THEREBY TRANSMITTED TO THE SURFACE OF A WORK PIECE FORCING IT TO CONFORM TO THE CONFIGURATION OF A DIE, SAID WORK PIECE AND SAID DIE BEING IMMERSED IN SAID FLUID; THE IMPROVEMENT WHICH COMPRISES SUBSTITUTING FOR SAID FLUID A LIQUID HAVING A TEMPERATURE OF -100*F. OR LESS AND ALLOWING THE TEMPERATURE OF SAID WORK PIECE TO COME TO EQUILIBRIUM WITH THE TEMPERATURE OF SAID LIQUID BEFORE FORMING SAID WORK PIECE. 